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Psv Sizing Flowrate? Actual Vs Design

psv psv sizing relief device relief device sizing prv prv sizing overpressure overpressure scenario

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#1 Tom Berenger

Tom Berenger

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Posted 07 June 2018 - 02:58 PM

Hi experts, 
I have a few questions about PSV sizing. I got some drawings and datasheets from a vendor for a fuel gas skid and I don't think the vendor calculations are correct. I will appreciate if you can help me with answers to the following questions:
(1) Fuel gas skid is designed for a maximum flowrate of 2 mmscfd BUT the actual usage is 0.5 MMscfd. Do you size PSVs based on 2 mmscfd figure OR 0.5 mmscfd figure? Keeping in mind that usage might increase due to possible plant expansion.
(2) When sizing PSV for the fuel gas scrubber, do you size based on flowrate OR do you size based on scrubber volume?
I have always sized PSVs based on actual flowrate to prevent oversizing and/or compromising integrity of the valve (e.g. chattering)
You inputs are welcomed and appreciated.

#2 Bobby Strain

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Posted 07 June 2018 - 03:11 PM


No one can provide an appropriate answer given such little information.



#3 fallah


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Posted 08 June 2018 - 06:50 AM

Lack of adequate info...as Bobby well mentioned...

#4 Pilesar


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Posted 08 June 2018 - 09:05 AM

The PSV is usually the last safety system before a catastrophe occurs. If the PSV is not large enough, what will you tell the family of the deceased? Design the PSV to handle every reasonable scenario for overpressure. This means sizing for maximum flow rate, fully open control valves and bypass valves, maximum inlet pressures, etc. If there is something in your plant that physically constrains the flow (such as small piping, restriction orifices, compressor limitations) you can consider these as part of your safety system ONLY if they cannot be easily bypassed. If you are in charge of PSV calcs, there may not be a rigorous Management of Change implementation at your site. Could someone put in larger piping or control valves without re-evaluating the PSV? This is another reason to be extra conservative.

  To address your question two, change the OR to an AND. Size the PSV for EVERY reasonable overpressure scenario and choose the design case that requires the largest PSV. Design the plant controls so that the relief valves never lift.

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